Re: What is correct way to describe this in BNF for an LL(1) parser

Thanks to both of you for your help.


The major problem appears to be in the parser that I'm using. I created
a minimal language :


expression term term-tail
term-tail addition term-tail
term-tail
addition + term
addition term
term name
term | expression |


and then traced what the parser was doing (I'm using this parser and
run-time because I have the sources available to me).


Given the simple input sentence


|a|


the parser tun-time was correctly working until it matched the 'a' with
the first 'term' production. It then looked to see what would match the
second input "|" and decided that the path '1st term-tail, 2nd
addition, 2nd term' productions were the go, thinking that it was the
start of a second term with am implicit operator. It then found the end
of the input and flagged an error.


I think this is because of an 'exception' the parser allows to strict
LL(1) rules (for associative binary operators such as '+' where it
allows the single operator to have more than 2 arguments in the
abstract syntax tree - what it calls "bushy trees"). When it has the
choice of a production with the appropriate head symbol and possibly
passing back through some 'stacked' (partially parsed) productions that
can be empty, it will always choose the non-empty production. In this
case it is where the whole thing is going wrong.


So, thanks again - I guess its back to the debugger and 'thinking cap'
to 'fix' (probably remove) this 'feature'


Regards


Don

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